Young children exhibit several deficits in reasoning about their own and other people's mental states. We propose that these deficits, along with more subtle limitations in adults' social-cognitive reasoning, are all manifestations of the same cognitive bias. This is the 'curse of knowledge' - a tendency to be biased by one's own knowledge when attempting to appreciate a more naive or uninformed perspective. We suggest the developmental differences in mental state reasoning exist because the strength of this bias diminishes with age, not because of a conceptual change in how young children understand mental states. By pointing out the common denominator in children's and adults' limitations in mental state reasoning we hope to provide a unified framework for understanding the nature and development of social cognition.